Remove view-mode toggle, AppHeader, and StatusBar; collapse to split layout
The view-mode switcher offered overview vs. split, but the diff panel is
core to the desktop client, so the overview-only branch was dead weight.
Drop the mode concept entirely (atom, type, persisted layout field on
both TS and Rust sides, navigation hook view-mode coupling, and the
1/2 keyboard shortcuts) and always render the split layout.
Move the workspace switcher, revision search, and sync into the
revisions sidebar so the AppHeader can go away. The macOS title bar
handles window dragging now that the in-app drag region is gone.
Drop the StatusBar — its theme toggle now lives as Light/Dark/System
entries under the new Appearance group in the command palette. The
shortcut help button is redundant since the ? key already opens the
help dialog. Closest-bookmark BFS and the working-copy status query
were only feeding the StatusBar, so they go too.
Empty state keeps just the workspace switcher above the EmptyState card.